Apparatus and method for optical switching with liquid crystals and birefringent wedges

ABSTRACT

An optical switching apparatus includes at least one optical waveguide to deliver at least one input optical beam. A dispersion device spatially separates the at least one input optical beam into individual wavelength channels. An optical power device aligns the individual wavelength channels. An optical switch has at least one transflective polarizing element, at least one birefringent wedge and at least two polarization switches. The individual wavelength channels are directed to independently addressable regions of the polarization switches for wavelength selective switching. A second optical power device aligns the individual wavelength channels from the optical switch. A second dispersion device spatially combines individual wavelength channels from the second optical power device. At least one output optical waveguide receives at least one of the individual wavelength channels from the second dispersion device.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of “Apparatus and Method forOptical Switching with Liquid Crystals and Birefringent Wedges”, U.S.Ser. No. 11/318,068, filed Dec. 23, 2005, which claims the benefit ofU.S. Provisional Patent Application entitled “A 1×M Optical SwitchUtilizing Liquid Crystals and Birefringent Wedges”, Ser. No. 60/639,107,filed Dec. 23, 2004.

FIELD OF THE INVENTION

The present invention relates to optics. More particularly, theinvention relates to beam routing in wave-guided optical systems.

BACKGROUND OF THE INVENTION

A dense wavelength division multiplexed (DWDM) optical network, as withany information network, requires switches to perform routing ofsignals. DWDM networks pass several information channels along the sameoptical waveguide (e.g., optical fiber). Each channel corresponds to adifferent wavelength of light with the wavelengths typically separatedby less than a nanometer. Consequently, DWDM networks require switchesthat are wavelength selective with very high resolution.

Regarding terminology, a 1×M optical switch comprises one input fiber(or port) and M output fibers (ports) to which the input can beselectively routed. A 1×M wavelength-selective switch similarly has oneinput port and M output ports and the capability of directing each of anumber of discrete wavelengths from the input to any of the M outputports. The techniques of the invention may also be used to support Minput fibers (ports) and I output fiber (port). Further, the techniquesof the invention may be used to construct a system with M input fibers(ports) and M output fibers (ports).

For wavelength-selective optical switching, two commonly employedswitching elements are micro-electromechanical (MEM) mirrors and liquidcrystals (LC). These technologies use free space optics: the opticalsignal is removed from the fiber waveguide, manipulated using unguidedoptical components and then reinserted into an output fiber waveguide.Waveguided approaches (e.g., planar light circuits or PLCs) have beenproposed for such functions but to date their promise has not beenrealized because of technical problems remaining to be overcome.

MEM micromirrors are constructed using microlithographic techniques. Themirrors are deformed or reoriented using electrostatic forces. Becauseof their small size and method of fabrication, it is straightforward toproduce the arrays of mirrors required for wavelength-selectiveswitching. Also, because the mirrors can take on a range of orientationsthey are conceptually easy to implement for higher port countwavelength-selective switches. It is the flexibility of the beamsteering mechanism that makes MEM devices so promising and at the sametime creates significant challenges for control and long term stability.MEM devices rely on steering a reflected beam; controlling the angle ofreflection is paramount. Small deviations (<0.1 degree) in signaldeflection can dramatically increase the coupling losses to an outputport. Fabrication of the MEMs arrays requires an expensive processingfacility, which makes them a costly solution for low volumeapplications.

Liquid crystal (LC) technology has a relatively long history in theprior art for optical switching applications. Liquid crystals are fluidsthat derive their anisotropic physical properties from the long rangeorientational order of their constituent molecules. Liquid crystalsexhibit birefringence and the optic axis of a LC fluid can be reorientedby an electric field. This switchable birefringence is the mechanismunderlying all applications of liquid crystals to optical switching andattenuation.

Two mechanisms have been proposed in the prior art for optical switchingusing liquid crystals: polarization modulation and total internalreflection (TIR). This refers to signal redirection to one of at leasttwo channels (1×M switch; M>1). On/off liquid crystal optical switchescan also be constructed on the principle of switchable scattering.

TIR liquid crystal switches rely on the difference in refractive indexbetween the liquid crystal and the confining medium (e.g., glass). Byproper choice of materials and angle of incidence of the light at theliquid crystal interface, it is possible to totally internally reflectthe light when no field is applied to the liquid crystal. The effectiveindex of the liquid crystal may be changed by reorienting the optic axisof the liquid crystal so that the total internal reflection criterion isno longer met; light then passes through the liquid crystal rather thanreflecting from the interface. As with other types of reflectivedevices, such as MEM devices, controlling the reflection angle iscritical. Also, since unwanted surface reflections are always present tosome degree, crosstalk can be a significant problem.

Polarization modulation is the most common mechanism used in liquidcrystal devices for optical switching. Switching is achieved between twoorthogonal polarization states: for example, two orthogonal linearpolarizations or left and right circular polarization. By way ofillustration, a simple liquid crystal polarization modulator is shown inFIG. 1. The structure of the device is shown in cross-section FIG. 1 a.A layer of nematic liquid crystal 1 is sandwiched between twotransparent substrates 2 and 3. Transparent conducting electrodes 4 and5 are coated on the inside surfaces of the substrates. The electrodesare connected to a voltage source 6 through an electrical switch 7.Directly adjacent to the liquid crystal surfaces are two alignmentlayers 8 and 9 (e.g., rubbed polyimide) that provide the surfaceanchoring required to orient the liquid crystal. The alignment is suchthat the optic axis of the liquid crystal is substantially the samethrough the liquid crystal and lies in the plane of the liquid crystallayer when the switch 7 is open. FIG. 1 b depicts schematically theliquid crystal configuration in this case. The optic axis in the liquidcrystal 10 is substantially the same everywhere throughout the liquidcrystal layer. FIG. 1 c shows the variation in optic axis orientation 12as a result of molecular reorientation that occurs when the switch 7 isclosed. The liquid crystal cell as described is known in the field as anelectrically controlled birefringence device (or ECB). Such a liquidcrystal polarization modulator was described in U.S. Pat. No. 5,276,747as part of an optical switch/variable optical attenuator (VOA) for fiberoptic communications applications.

To act as a switch, the modulator must produce two orthogonalpolarizations at the exit of the modulator that can then bedifferentiated with additional optical components. This polarizationconversion scheme provides the foundation for a number of electro-opticdevices. If a linear polarizer is placed at the exit to the modulator, asimple on/off switch is obtained. If a polarizing beam splitter isplaced at the exit, a 1×2 switch can be realized.

As an example, we consider a switchable half wave retardation plate. Forthis case, the liquid crystal layer thickness, d, and birefringence, Δn,are chosen so that

$\begin{matrix}{\frac{\Delta\;{nd}}{\lambda} = \frac{1}{2}} & (1)\end{matrix}$where λ is the wavelength of the incident light. In this situation, withreference to FIG. 1 b, if linearly polarized light with wave vector 13is incident normal to the liquid crystal layer with its polarization 14making an angle 15 of 45 degrees with the plane of the optic axis 10 ofthe liquid crystal, the light will exit the liquid crystal linearlypolarized with its polarization direction 16 rotated by 90 degrees fromthe incident polarization 14.

Referring now to FIG. 1 c, the optic axis in the liquid crystal isreoriented by a sufficiently high field. If the local optic axis in theliquid crystal makes an angle Θ with the wave vector k of the light, theeffective birefringence at that point is

$\begin{matrix}{{{\Delta\; n_{eff}} = {\frac{n_{e}n_{o}}{\sqrt{{n_{o}^{2}\cos^{2}\Theta} + {n_{e}^{2}\sin^{2}\Theta}}} - n_{o}}},} & (2)\end{matrix}$where n_(o) and n_(e) are the ordinary and extraordinary indices of theliquid crystal, respectively. The optic axis in the central region ofthe liquid crystal layer is nearly along the propagation direction 13.In this case, according to Eq. 2, both the extraordinary 17 and ordinarycomponents 18 of the polarization see nearly the same index ofrefraction. Ideally, if everywhere in the liquid crystal layer the opticaxis were parallel to the direction of propagation, the medium wouldappear isotropic and the polarization of the exiting layer 19 would bethe same as the incident light 14.

Besides the ECB device described above, a number of other liquid crystaldevices can operate as polarization switches: 90° degree twistednematic, 270° twisted nematic, and ferroelectric LC are three commonexamples.

While it is easy to conceptualize a 1×2 LC-based switch, unlike the MEMdevice technology, generalization to 1×M with M>2 is problematic becauseof the discrete 2-state nature of the polarization switching. Cascadingschemes have been proposed for broadband signal routing; however, theseapproaches are not amenable to wavelength-selective switching in a DWDMnetwork.

The liquid crystal device of FIG. 1 is appropriate for broadband signalrouting. However, for wavelength-selective switching, a liquid crystaldevice with more than one independently switchable element (pixel) isuseful. A 1×N linear array of N separately addressable pixels isillustrated in FIG. 2. The structural cross-section of this LC array isidentical to that of FIG. 1 a. The liquid crystal is confined betweentwo glass substrates 202 and 204 by means of a seal 206. Substrate 202has one continuous electrode 208 which serves as the common electrodefor the pixels. Substrate 204 has an array of N photolithographicallypatterned electrodes 210. Each pixel 212 is defined by the region ofoverlap between the common electrode and one of the patternedelectrodes. By applying an independent voltage to each patternedelectrode, the electro-optic response of each pixel in the array can beseparately controlled.

SUMMARY OF THE INVENTION

The invention includes an optical switching apparatus with at least oneoptical waveguide to deliver at least one input optical beam. Adispersion device spatially separates the at least one input opticalbeam into individual wavelength channels. An optical power device alignsthe individual wavelength channels. An optical switch has at least onetransflective polarizing element, at least one birefringent wedge and atleast two polarization switches. The individual wavelength channels aredirected to independently addressable regions of the polarizationswitches for wavelength selective switching. A second optical powerdevice aligns the individual wavelength channels from the opticalswitch. A second dispersion device spatially combines individualwavelength channels from the second optical power device. At least oneoutput optical waveguide receives at least one of the individualwavelength channels from the second dispersion device.

The invention also includes an optical switch with an ingresspolarization switch, a transflective polarizing element, and an assemblyincluding at least one birefringent wedge and at least two polarizationswitches. The ingress polarization switch is operative to produce afirst polarization state and a second polarization state. Thetransflective polarizing element transmits the first polarization stateand reflects the second polarization state. Individual wavelengthchannels are directed to independently addressable regions of the atleast two polarization switches for wavelength selective switching.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is more fully appreciated in connection with the followingdetailed description taken in conjunction with the accompanyingdrawings, in which:

FIG. 1 a is a schematic drawing of a prior art electrically-drivenliquid crystal cell that may be used as a polarization rotator in anembodiment of the current invention;

FIG. 1 b is a schematic illustrating the rotation of the polarization oflinearly polarized light by 90° upon passage through the liquid crystalcell of FIG. 1 a when no voltage is applied to the cell.

FIG. 1 c is a schematic illustrating no rotation of the polarization ofincident linearly polarized light upon passage through the liquidcrystal cell of FIG. 1 a when sufficiently high voltage is applied tothe cell.

FIG. 2 is a schematic illustration of a prior art liquid crystal cellcontaining an array of pixels which can be independently activated by anapplied voltage.

FIG. 3 is a schematic illustration of the liquid crystal andbirefringent wedge assembly of the current invention. An input beam oflinearly polarized light passing through the assembly with M LC cellsand M wedges can be selectively directed into any one of 2^(M) outputdirections.

FIG. 4 a illustrates a prior art birefringent wedge whose optic axis isorthogonal to the sides of the wedge.

FIG. 4 b illustrates the effect that the wedge of FIG. 4 a has onincoming polarized light. Light polarized parallel to the optic axis isdeflected at a larger angle from the direction of the incident beam thanlight polarized orthogonal to the optic axis.

FIG. 5 a is a detailed schematic of a single stage of the LC/wedgeassembly of FIG. 4. A vertically polarized incident beam is converted tohorizontally polarized light by the LC cell in its low voltage state andis subsequently deflected by the birefringent wedge. The optic axis ofthe wedge is oriented as in FIG. 3 so that the polarization of the lightis parallel to the optic axis.

FIG. 5 b is the same as FIG. 5 a except that the LC cell is in its highvoltage state and the polarization of the incident light as unchanged bythe cell. In this case, the polarization of the beam passing through thebirefringent wedge is perpendicular to the optic axis and the beam isdeflected less than the case in FIG. 5 a.

FIG. 6 a is a side view of a prior art structure for converting anarbitrarily polarized beam from an optical fiber into two parallel beamswith identical polarization.

FIG. 6 b is an end-on view of the prior art structure of FIG. 6 ashowing the orientation of the optic axis of the half wave plate used toconvert the polarization of the extraordinary ray to that of theordinary ray.

FIG. 7 a is bifurcation diagram for a 1×4 switch according to anembodiment of the invention when the wedge angles for the two wedges areidentical.

FIG. 7 b is bifurcation diagram for a 1×4 switch according to anembodiment of the invention when the wedge angles for the two wedges areθ and 2θ.

FIG. 8 is a schematic diagram showing the operating principal of a priorart Wollaston polarizer.

FIG. 9 is an embodiment of a broadband 1×4 optical switch according toan embodiment of the current invention.

FIG. 10 a is an embodiment of a 1×4 wavelength selective switchaccording to an embodiment of the current invention which operatesexclusively in transmission.

FIG. 10 b shows a detail of the LC/wedge switching assembly for theswitch of FIG. 10 a.

FIG. 10 c is a detailed schematic of the output port fiber couplingoptics for the switch of FIG. 10 a.

FIG. 11 a is an embodiment of a 1×4 wavelength selective switchaccording to an embodiment of the current invention which operatesexclusively in reflection.

FIG. 11 b shows a detail of the LC/wedge switching assembly for theswitch of FIG. 11 a.

FIG. 11 c is a detailed schematic of the input/output port fibercoupling optics for the switch of FIG. 11 a.

FIG. 11 d illustrates output beams sharing a fiber coupling assembly.

FIG. 12 is the bifurcation diagram for the embodiment of FIG. 11.

FIG. 13 a is an embodiment of a 1×4 wavelength selective switchaccording to an embodiment of the invention that operates partly inreflection and partly in transmission through the inclusion, in theswitching assembly, of a polarizer which reflects one linearpolarization and transmits an orthogonal polarization.

FIG. 13 b shows a detail of the LC/wedge switching assembly for theswitch of FIG. 9.

FIG. 13 c is a detailed schematic of the input/reflected output portfiber coupling optics for the switch of FIG. 9.

FIG. 13 d is a detailed schematic of the transmitted output port fibercoupling optics for the switch of FIG. 9.

FIG. 14 is a schematic of a collimator holder array according to anembodiment of the invention.

FIG. 15 is measured optical performance data for a 1×4 wavelength switchaccording to an embodiment of the current invention.

FIG. 16 a shows a detail of the LC/wedge/transflective-polarizerswitching assembly for an embodiment of a 1×5 wavelength selectiveswitch that contains two reflecting elements for directing beams todifferent output ports.

FIG. 16 b is a schematic of the input/output port fiber coupling opticsfor an embodiment of a 1×5 wavelength selective switch that contains tworeflecting elements for directing beams to different output ports.

FIG. 17 is a schematic of a 1×5 wavelength selective switch operating asa 4 port reconfigurable drop module in a fiber optic network.

Like reference numerals refer to corresponding parts throughout theseveral views of the drawings.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 3 is a schematic illustration of the operation principle of thecurrent invention. The switch comprises a number M of liquid crystalpolarization switches interleaved with M birefringent wedges. Referringto the figure, linearly polarized light 302 is incident from the leftand passes in serial fashion through LC Switch 1 304, Birefringent Wedge1 306, LC Switch 2 308, Birefringent Wedge 2 310, and so on until itexits from Birefringent Wedge M 312. Depending on the state of each ofthe LC switches, the output beam is deflected into a particular one of2^(M) directions 314.

To understand this more clearly, refer to FIG. 4. FIG. 4 a is aperspective view a birefringent wedge 402. In this figure, the opticaxis of the birefringent material 404 is indicated as lying in thehorizontal plane when the apex of the wedge points vertically. That is,it is parallel to the vertex edge of the wedge. It is not a requirementof this invention that the optic axis be so oriented, but it is chosenfor illustrative purposes in elucidating an embodiment of the invention.FIG. 4 b illustrates the impact that such a birefringent wedge has on abeam of polarized light passing through it. If the incident beam 406 haspolarization 408 parallel to the optic axis (i.e. an extraordinary ray),the action of the wedge is to deflect the beam away from the vertex uponexit. The deflection angle depends substantially linearly on theextraordinary index of refraction, n_(e), of the wedge and the wedgeangle θ. On the other hand, if the incident beam has polarization 410orthogonal to the optic axis (i.e. an ordinary ray), the deflectionangle upon exit will depend on the ordinary index, n_(o), andconsequently there will be an angular different φ 412 between theordinary 414 and extraordinary rays 416 upon exiting the wedge. Thisseparation angle φ depends substantially linearly on the wedge angle θ418 and the birefringence, n_(e)-n_(o), of the wedge. Of course, if theinput polarization is a combination of both polarizations, the inputbeam will be partially diverted into both exit directions. This is notdesirable for a switch application where the beam should be routed intoeither one or the other of the two directions. FIG. 4 b presumes thatthe extraordinary index of the wedge is great than the ordinary index(n_(e)>n_(o)) resulting in a greater deflection of the extraordinaryray. If n_(o)>n_(e) then the ordinary ray will have the greaterdeflection. To avoid confusion, all examples and embodiments assume thatn_(e)>n_(o) but note that this is not a requirement of the invention.

FIG. 5 illustrates the operation of the first stage of the LC/wedgeassembly of FIG. 3. The wedge is presumed to have the same optic axisorientation as in FIG. 4 with n_(e)>n_(o). Referring to FIG. 5 a, a beamof light is incident from the left on the LC switching cell. Theincident beam 302 is linearly polarized in the vertical direction 508.Upon passing through the LC switch cell 304 in its low voltage state(electrical switch 511 open), the polarization 510 is rotated 90° sothat it passes through the birefringent wedge 306 as an extraordinaryray and is deflected accordingly. Referring now to FIG. 5 b, the sameincident beam when passing through the LC, here in its high voltagestate (electrical switch 511 closed), experiences no polarization changeand passes through the wedge as an ordinary ray and is deflected througha smaller angle than for the low voltage state of the LC. Hence, LCSwitch 1 and Birefringent Wedge 1 produce two possible output directions512 and 516 for the incident beam as indicated in FIGS. 5 a and 5 brespectively. Each of these output beams can be steered into two furtherdirections by the action of LC Switch 2 and Birefringent Wedge 2,resulting in 4 possible beam propagation directions after the secondstage of the assembly. Continuing in similar fashion, for an assembly ofM stages, there are 2^(M) possible output propagation directions for theexit beam.

This preceding discussion gives a conceptual overview of the inventionbut ignores some significant details that are necessary to produce auseful device for routing or wavelength selective switching in a DWDMfiber optic network.

First, in a fiber optic network, the light does not have a controlledpolarization. This results from polarization modification by opticalcomponents in the system (e.g. optical amplifiers, gain equalizers,attenuators) as well as ubiquitous form and strain birefringence in thefiber itself. Hence, the LC/wedge assembly described above is useless insuch a network unless a means is provided to achieve a well-defined,controlled polarization for the optical beam prior to entering theswitch assembly. This is, of course, a common problem for whichsolutions have been described in the prior art.

FIG. 6 illustrates perhaps the most widely used means to address thisproblem. Referring to FIG. 6 a, light exits an optical fiber 602 andpasses through a system with optical power (a collimator) 604 whichcollimates the light into a beam 606 of arbitrary polarization 608. Thisbeam is passed through a birefringent crystal 610 of sufficient lengthand proper optic axis orientation 612 to separate the ordinary 614 andextraordinary 616 beams sufficiently so that they do not overlap at theexit surface of the crystal. (In such an application, the birefringentcrystal is known to those familiar with the art as a beam displacer (BD)or a walkoff crystal.) One of the beams (shown as the extraordinary beamin FIG. 6 a) is then passed through a half wave retardation plate 618which rotates the beam's polarization by 90° so that there are twoparallel beams 620 with identical and well-defined linear polarization.FIG. 6 b is an end view of the crystal showing the orientation of theoptic axis 622 of the half wave retardation plate which produces thedesired 90° rotation of polarization for the optical system as presentedin FIG. 6 a. This scheme operates also in reverse so that two parallelbeams of identical polarization can be combined and coupled into anoptical fiber using the same configuration of elements. Henceforth, theoptical assembly as shown in FIGS. 6 a and 6 b and described above shallbe referred to as a fiber coupling assembly, whether it be at the inputor output of a fiber.

Unlike prior art switches using liquid crystals and beam displacerswhich produce a spatial separation of parallel beams, this inventionproduces an angular discrimination. Consequently, the wedge angles arechosen so that beams corresponding to different switch states exit theswitching assembly at sufficiently separated angles to bedistinguishable. As an example, consider a 1×4 switch with two LCswitches and two birefringent wedges. For concreteness, here we considerboth stages of the switch to be configured as in FIG. 5. Then the opticaxes of the wedges are parallel, and we refer to light polarizedparallel to this direction as e-polarized. If the wedges are identical,two of the switch states will produce the typically undesirablesituation of two of the four possible beams propagating in the samedirection upon exit from the switching assembly. This is illustrated inFIG. 7 a which is the switching bifurcation diagram for this situation.The bifurcation diagram is a pictorial representation of the impact theswitching assembly has on an input beam of light. Referring to FIG. 7 a,light 702 of well-defined polarization, here shown as e-polarized forpurposes of illustration, is input from the left and passes through thetwo stages 704 and 706 of the switch to the output. Each stage comprisesthe operations of a switch cell and a birefringent wedge. At theentrance to each stage there is one or more switching nodes; the firststage has one input node 708 and the second has two input nodes 710. Thetwo lines emanating from each node represent the deflections of the twoorthogonal polarizations (e-polarization and o-polarization) selectableby each switch cell. Vertical displacement represents relative angularseparation of the different possible beam trajectories through theassembly. In that sense, only the vertical positions at the exit of eachstage has physical meaning. However, if the lengths of the stages areequal in the diagram, the angle between the two lines emanating fromeach node will be proportional to the angle φ between the e-ray ando-ray as in FIG. 4 b. (This is true as long as the wedge angle θ issmall enough so that sin θ≈θ.) Each continuous path from the input tothe output represents a state of the switch. We can designate each pathby the polarization of the light in each stage. For example, (e,e)indicates the path traversed when the beam is e-polarized in both stagesof the switch. Using this notation, we observe from FIG. 7 a that paths(e,o) and (o,e) exit the switch assembly at the same angle since theyintersect at an exit node 712.

To obtain equal angular separation between the 4 beams, if one wedge haswedge angle θ, the other wedge will have a wedge angle of 2θ againprovided that sin θ≈θ. This is illustrated by the bifurcation diagram ofFIG. 7 b, which shows the 4 exit nodes 714 equally spaced in thevertical direction. In FIG. 7 b, the wedge with the larger wedge angleis in Stage 1 704. However, the order of the wedges does notsubstantially affect the relative angular displacement of the outputbeams, although beams with the same angular deflection will correspondto different switch states if the wedges are interchanged.

Finally, we observe that, as FIG. 4 illustrates, both the e-ray ando-ray are deflected away from the birefringent wedge vertex and notsymmetrically with respect to the input beam direction. If the wedgesare all oriented the same, upon passage through successive stages of theswitch, the deflected beams will be steered further from the input beamdirection. This may be undesirable for certain switch geometries, and inparticular, is extremely detrimental to the design of a 1×M wavelengthswitch. This problem can be mitigated in a few ways. One simple way tolessen the deflection for more than one stage is to alternate theorientation of the wedges, so that the vertices point in oppositedirections. This will lessen the deflection, but cannot produce M beamsuniformly distributed about the incident direction. Another means whichcan produce such a uniform distribution is a wedge made of isotropicmaterial. A third approach is to replace a birefringent wedge with abirefringent wedge pair whose optic axes 802 and 803 are orthogonal asillustrated in FIG. 8. This configuration is known in the art as aWollaston polarizer. It has the property that for a normally incidentbeam 804, the beam is split into two orthogonally polarized beams 806and 808 whose deviations are symmetric with respect to the incidentdirection of propagation. To obtain the same angular deviation φ 810between the two beams as is achieved in the single wedge case, the wedgeangle 812 for each member of the Wollaston pair should be half of thatfor the single wedge design.

FIG. 9 illustrates a first embodiment of the invention. It is a 1×4optical routing switch for a fiber optic system. It is not wavelengthspecific. Light exits the input fiber 902 and passes through acollimator/BD/retarder coupling assembly 904. The two parallel beamsthen pass through the LC/wedge switching assembly 906 and through theaction of the switch the beams are deflected to 1 of 4 output ports 908.Each output port contains a retarder/BD/collimator assembly 910 forcoupling to that port's output fiber. The switch assembly is forconcreteness here assumed to have its LC and wedge configurationaccording to the bifurcation diagram of FIG. 7 b. With reference to FIG.7 b, it is clear that two of the beams output from the switch assemblyhave their polarizations orthogonal to the input polarization. For thesetwo beams, which correspond to output ports 1 912 and 3 914 in FIG. 9,the half wave retarders 916 and 918 in the coupling assembly must bemoved, as shown in the figure, to the opposite beam in the pair fromthat of the retarder at the input coupling assembly. Otherwise, thesebeams will not couple into the fiber.

FIG. 10 illustrates a second embodiment which functions as a 1×4wavelength switch for a WDM fiber optic network. With reference to FIG.10 a, this device comprises (1) an input fiber coupling assembly 1002(i.e. collimator/BD/waveplate) which provides two parallel beams ofidentical polarization, (2) a dispersive means 1004 (e.g., a grating)which takes these polarized beams and separates them into theircomponent wavelengths 1006, (3) a means with optical power 1008 (e.g. alens) in the path of the dispersed beams which serves two functions: itconverts the diverging dispersed beams into an array of parallel beamsand focuses the beams on the switching assembly, (4) an LC/wedgeswitching assembly 1010, (5) a second means with optical power 1012(lens) that performs the inverse functions of the first means withoptical power, collimating the dispersed beams and focusing these beamsto the same point on (6) a second dispersive means 1014 (e.g., a secondgrating) which combines the dispersed beams into one or more pairs ofparallel beams which are directed to (7) an array of output couplingoptics 1016 for connecting each pair of beams to one of the output portfibers 1018.

A side view detail of the switching assembly is shown in FIG. 10 b. Thewedge angles of the birefringent wedges 1020 and 1022 are θ and 2θrespectively, in order of passage by the light. This is the reverse ofthe situation described earlier with reference to the bifurcationdiagram of FIG. 7 b and results in different routing paths.(Additionally, here one of the wedges is inverted.) Beam path (o,e) goesto Port#1 1024; path (e,o) goes to Port #2 1026; path (e,e) goes to Port#3 1028; and path (e,o) goes to Port #4 1030. Since the two e-polarizedexit beams go to Ports #1 and #3, the half wave retardation plates onthe coupling assemblies for these two ports (1032 and 1034,respectively) must be reversed from that of the input as shown in thedetail of the output coupling assembly array (FIG. 10 c). As notedearlier and practiced here, the two birefringent wedges have their wedgeangles opposed to reduce steering of the beam either toward or away fromthe optical mounting base 1019. A third (isotropic) wedge 1031 is alsoincluded, as illustrated in FIG. 10 b to adjust the output beams so thatthey are symmetrically distributed about the centerline of the opticalsystem. Regarding the wedge angle θ of the birefringent wedges, it mustbe chosen large enough so that the beams traveling to the differentports are sufficiently separated that light intended for one port is notcaptured by an adjacent port (i.e., good port isolation). Thisundesirable coupling is known as port crosstalk. The required angle θdepends on the beam diameter as well as the focal length of the lens.Generally speaking, the minimum allowable θ to achieve the requiredperformance will vary directly with the beam diameter and inversely withthe focal length of the lens.

Each liquid crystal cell in FIG. 10 b comprises a 1×N array of pixels asin FIG. 2, one pixel for each of the N wavelengths in the multiplexedsignal. The two LC cells have their pixel arrays aligned such that aparticular wavelength λ_(k), passes through the k^(th) pixel in botharrays, where k is an integer from 1 to N. Every pixel in both arrays isindividually drivable with a voltage, so that each wavelength can beindependently steered to any one of the 4 output ports.

Before leaving this embodiment of a wavelength-selective routing switch,note that for proper operation of the device, the two dispersive meansas well as the two means with optical power should be opticallyidentical or at least very nearly so. It has been taught in the priorart that if this is not the case, it will not be possible to multiplexthe demultiplexed beams and couple them efficiently into the outputports. It has been further taught that not only must these elements beidentical, they must be oriented very precisely with respect to eachother. More specifically, they must have mirror symmetry with respect toa plane which is midway between the two means with optical power andoriented normal to the line joining the centers of these means. Thismakes system alignment very sensitive. A reflective design which usesthe same dispersive means and the same means with optical power for boththe input and output stages can remove much of this alignmentsensitivity.

FIG. 11 is a third embodiment of a 1×4 wavelength-selective switch thatcontains a reflective means, thereby eliminating the second means withoptical power as well as the second dispersive means of the previousembodiment. With reference to FIG. 11 a, light containing N discretewavelengths exits the input fiber 1102, passing first through an inputcoupling assembly 1104 which produces two parallel beams with the samepolarization. A dispersive means 1108 (here a diffraction grating)separates the beams into N pairs of beams 1110, one pair for eachcomponent wavelength. A means with optical power 1112 (here a convexspherical lens) focuses the separate beams onto the LC switch assembly1114. A reflective means after the switch assembly then returns thelight in reverse order back through the LC assembly, the means withoptical power, and the dispersive means after which it is coupled backto 1 of 4 four output ports via the coupling array. A detail of theswitching assembly is illustrated in FIG. 11 b. Each liquid crystal cell1116 in FIG. 11 b comprises a 1×N array of pixels as in FIG. 2, onepixel for each of the N wavelengths in the multiplexed signal. The twoLC cells have their pixel arrays aligned such that a particularwavelength λ_(k), passes through the k^(th) pixel in both arrays. Everypixel in both arrays is individually drivable with a voltage, so thateach wavelength can be independently steered to any one of the 4 outputports. In this embodiment, the wedge angles of the birefringent wedges1118 are 2θ and θ respectively, in order of passage by the light. Noisotropic prism or other correction means is required for beam steeringbecause the mirror 1120 in the switch assembly can be tilted to directthe reflected beams back along the desired path. FIG. 11 c shows therelative positions of input beam 1122 and return beam paths 1124 throughthe lens and switching assembly as determined by the tilt of the mirroras in FIG. 11 b. In particular, the mirror angle has been chosen in thisembodiment so that the input beam and the reflected beam for Port #3overlap. This overlap is not a requirement but offers the advantage ofminimizing the overall height of the system. The input and Port #3output beams consequently share the same fiber coupling assembly asillustrated in FIG. 11 d. An optical circulator 1126 is added to thisport to separate the Port #3 output 1128 from the input 1130 as shown inFIG. 11 d. Another advantage of this configuration is that any lightbeam directed into one of the ports 1, 2, and 4 will, with theappropriate selection of switch voltages, retrace the paths outlinedabove and will exit through Port #3. The same switch voltages that allowthe input beam from Port #3 to be directed to each of the ports 1, 2 or4 will correspondingly direct any wavelength coming into those ports tobe directed to output Port #3. Thus, adding circulators to any of theports 1, 2 or 4 will allow them to be used as both add and drop ports inan optical network.

Two additional points on the reflective embodiment can be made. First,because of the double pass of the light through the wedges, the wedgeangle θ is half of that of the transmitted embodiment describedpreviously to achieve the same port separation, provided that the beamwidths and lens focal lengths are the same for both embodiments.Secondly, again because of the double pass through the switchingassembly, the polarizations of all of the beams exiting the assembly areidentical. This point is illustrated in FIG. 12, the bifurcation diagramfor this embodiment. Hence, the configurations of the fiber couplingoptics for all of the ports are identical including the positions of thehalf wave retardation plates.

A fourth embodiment to be considered is a 1×4 wavelength selectiveswitch that incorporates both transmitted ports (in the sense of thesecond embodiment) and reflected ports (in the sense of the 3^(rd)embodiment) through the introduction of a transflective polarizer (i.e.a polarizer that transmits one linear polarization and retro-reflectsthe orthogonal polarization). Such transflective polarizers are extantin the art. Referring to FIG. 13 a, which is a schematic of thisembodiment, and comparing to FIG. 10 a, observe that from theperspective shown, there is little apparent difference between the twoembodiments. The differences occur in the structure of the switchassembly 1302, the input/output coupling array 1304 and the outputcoupling array 1306 that are required to produce two reflected ports1308 and two transmitted ports 1310. The differences are elucidated inFIGS. 13 b, 13 c and 13 d.

FIG. 13 b is a side view of the optical switching assembly for thisembodiment. As in the previous embodiment, there are two liquid crystalarrays 1314 with N elements for individually routing N wavelengths andtwo birefringent wedges 1312, the first with wedge angle 2θ and thesecond with wedge angle θ. A transflective polarizer 1316 is placedafter the second wedge; its transmitting axis is parallel to onepolarization of the light exiting the second wedge and orthogonal to theother. As with the previous embodiment, the polarizer is tilted todirect the two reflected beam pairs 1320 and 1322 backward through thesystem to the reflected output port fibers 1330 and 1332. An isotropicwedge 1318 is included after the polarizer to steer the beam pairs 1324and 1326 transmitted through the polarizer to the transmitted outputport fibers 1334 and 1336. It is advantageous, but not required, tochoose the transmitting axis of the polarizer to be parallel to thepolarization of the input beam as it enters the first LC cell. In thatsituation, the transmitted beams and the reflected beams have the samepolarization as the input upon exiting the switch assembly andconsequently all of the fiber coupling assemblies are identical. This isillustrated in FIG. 13 c for the input and reflected ports and FIG. 13 dfor the transmitted ports. With reference to FIGS. 13 c and 13 d, it isapparent that the spacing between the transmitted ports is half that ofthe reflected ports. This is a consequence of the double pass throughthe birefringent wedges for the reflected beams.

All of the embodiments described above for wavelength switching (i.e.embodiments 2 through 5) have their output collimators in a stackedconfiguration. Ideally, with perfectly aligned optics and the absence oflens aberrations, the orientation and spacing between the collimatorswould be identical. However, this is never the case in practice.Aligning and fixing the collimators in place is a critical step in thefabrication of these devices. Standard collimator holder arrays withfixed positions for each waveguide make this task extremely difficult,since each waveguide must be simultaneously aligned to the holder andthe optical beam. A solution to this problem is illustrated in FIG. 14.

FIG. 14 shows by way of illustration a 4 waveguide array where eachwaveguide 1402 is fixed in place by a set of structures 1404. Eachstructure has a common shape—a wedge in this example. For simplicity,each structure may also be of a common material, e.g., glass. In thisapproach, a waveguide 1402 is first aligned to the beam to optimize theoptical coupling. The collimator holder is then assembled around thewaveguide 1402 by placing the structures (e.g., wedges) 1404 in contactwith the waveguide 1402, as illustrated in FIG. 14. An adhesive isplaced on the wedges prior to assembly. The adhesive may be any adhesivecurable with ultraviolet light. Epoxy may be used as the adhesive. Thewedges provide large bond areas so minimal bond thickness may beemployed.

According to FIG. 14, four wedges per waveguide 1402 are used, althoughany number of wedges could be used. The collimator stack is compact andflexible with a strong structural shape when completed. The structurescan be stacked and provide for a constant nominal center-to-centercollimator spacing if the structures are all of the same thickness. Ifvariable collimator spacing is desired, structures of differentthickness can be used for each waveguide. Any number of waveguides canbe stacked in this manner. Using a spacer with an open slot, the firstwaveguide to be fixed does not have to be the bottom one in the stack.

This approach offers other key advantages over standard collimatorholder arrays, and for that matter, standard single collimator holders.For example, the wedges allow conformance to the waveguide along thelongitudinal axis of the waveguide. That is, each slanted surface ofeach wedge supports the waveguide along the longitudinal axis of thewaveguide that it is in contact with. The waveguides can be adjusted toa wide range of angles and displacements and still all the wedges willtightly conform to the waveguides along long narrow contact lines (e.g.,the interfaces between the longitudinal axes of the waveguides incontact with the slanted surfaces of the wedges). Additionally, sincethe wedges conform to the waveguide along a long narrow contact line,adhesive thickness between the waveguide and the holder is minimized.For example, if an adhesive with a relatively low viscosity is used, theadhesive will wick from an applied surface to the contact line with thewaveguide. This allows the adhesive bond thicknesses in the structuralpath to be minimal, on the order of microns. This minimizes the amountof fixing material with a coefficient of thermal expansion (CTE)significantly different from the waveguide. The wedges can be made froma material with nearly the same CTE as the waveguide and the entirestructural stack is made of close-packed pieces, mitigating strains inthe assembly over temperature which can substantially degrade theoptical coupling.

Also, because of the simple structure of the wedges, they can easily befabricated from a variety of materials. In particular, making the wedgesfrom glass allows ultraviolet curable epoxies to be used to fix thewaveguides, and one waveguide at a time can be aligned and fixed,although all the waveguides could also be fixed at the same time.Thermally cured epoxies could also be used including epoxies cured atroom temperature.

FIG. 15 shows the performance of an optical switch according to thethird embodiment: a reflective 1×4 wavelength switch. This figure showsthe output optical power versus wavelength measured at each port for allof the 4 possible switch settings. The wavelength range for operation ofthis device is from 1525 nm to 1570 nm. There is very little crosstalkbetween the ports; as is evident from the figure, the port isolation istypically >45 dB.

Those skilled in the art will recognize a number of benefits associatedwith the invention. For example, the invention provides an opticaldevice employing liquid crystals as active polarization switches toroute optical signals from an input optical fiber to one of a pluralityof output fibers. The invention also provides a system employing liquidcrystals as active polarization switches in conjunction withdemultiplexing and multiplexing means to route each wavelength in a DWDMnetwork from an input optical fiber to any one of a plurality of outputfibers.

Another embodiment of the invention is a 1×5 wavelength selective switchthat incorporates two reflective means in the LC assembly. Thisembodiment looks schematically identical to embodiment 3 (i.e., FIG. 10a). The differences occur in the LC cell assembly shown in FIG. 16 a andthe collimator/BD array shown in FIG. 16 b.

With reference to FIG. 16 a, the LC switch assembly has a 1×4 reflectiveassembly 1601 corresponding to the structure of FIG. 10 b plus twoadditional components: a third LC array 1602 and a transflectivepolarization 1603, placed in front of the 1×4 assembly. The LC array1602 is capable of switching the incident light between two orthogonalpolarization states. The transflective polarizer is oriented to transmitone of these polarizations and reflect the other. Consider first thecase where a pixel in array 1602 corresponding to a particularwavelength is switched so that the beam is transmitted through thetransflective polarizer. This transmitted beam then passes through the1×4 reflective assembly and according to embodiment 3 is directed to oneof four output ports 1604 (shown in FIG. 16 b), as selected by the twoLC arrays in assembly 1601 (shown in FIG. 16 a). In the opposite casewhere the beam is reflected by the transflective polarizer, it isdirected to an additional port 1605 (shown in FIG. 16 b). In this way,an input wavelength can be directed to any one of five output ports.

The advantage of this embodiment is singling out one port—the oneaddressed by reflection from the transflective polarizer. The lightdirected to this port is controlled only by the state of LC array 1602,whereas light transmitted to the remaining ports is controlled by thestate of all of the LC arrays. There are some situations where thisseparate control is desirable or necessary. One situation is where oneor more of the LC arrays is set to an intermediate state so that thelight exiting the LC array is in some intermediate state ofpolarization, which is a combination of the two orthogonal statesdiscussed previously. In that case, the beam corresponding to a givenwavelength is directed to 2 or more of the output ports simultaneously.This is illustrated with two example applications.

The first example is signal power monitoring in a fiber optic network.Power monitors are necessary elements in current DWDM networks. Forpower balancing of wavelengths, power must be monitored separately foreach wavelength. The current embodiment can serve this purpose if lightat port 1605 is coupled to a photodetector. By directing a small knownfraction (e.g., 1%) of only one wavelength at a time to this port, theoptical power associated with each wavelength can be determined byscanning sequentially through all pixels in LC array 1602. There is alarge savings in space and cost realized by integrating an optical powermonitor in this way.

As a second example, consider using the wavelength selective switch ofthis embodiment for dropping wavelengths at a node in an opticalnetwork. This is illustrated in FIG. 17. A fiber carrying multiplewavelengths of network traffic—the express fiber 1701—enters thewavelength selective switch, where each wavelength is either dropped to1 of 4 drop ports 1702 or passed to the express port 1703 and on to thenext node in the network. As stated above, it is possible to realizethis functionality with any of the embodiments 2-5 provided that nosignal is split between 2 or more ports. However, if the network isdesigned for broadcasting (i.e., dropping a signal at more than onenode), then the express signal must be split at one or more nodes. Ifthe drop port is arbitrary, only this last embodiment can realize thisfunctionality. Port 1605 is singled out as the express output port. Withthe pixel in LC array 1602 corresponding to the broadcast wavelength setto produce a mixed polarization state, part of the signal is reflectedto the express output; the remaining portion is passed to the 1×4assembly where it is directed to the desired drop port.

In addition to network broadcasting, this embodiment also supports nodebroadcasting (i.e., dropping a signal to all of the drop portssimultaneously) without effecting the express signal. This is done bysetting both LC arrays in the 1×4 assembly 1601 to produce a mixedpolarization state, sending portions of the signal to the 4 drop portswithout altering the signal on the express output. Embodiments 2-4 wouldalso broadcast to the express port as well as the drop ports, anundesirable constraint.

An LC array with the disclosed transflective polarizer may beincorporated into any of the embodiments 2-4 to achieve the samefunctionality.

Although the invention has been described in conjunction with particularembodiments, it will be appreciated that various modifications andalterations may be made by those skilled in the art without departingfrom the spirit and scope of the invention. Therefore, the inventionshould only be limited by the appended claims, wherein:

1. An optical switching apparatus, comprising: an optical switch with aningress polarization switch, a transflective polarizing element, and anassembly including at least M stages, where M is an integer with valueof two or more, each stage including at least one polarization switchand a single birefringent wedge to direct an input optical beam to oneof two output locations for further processing, the assembly therebyproviding 2^(M) output locations; wherein the ingress polarizationswitch is operative to produce a first linear polarization state and asecond linear polarization state; wherein the transflective polarizingelement transmits the first linear polarization state to the M stagesand reflects the second linear polarization state to a reflection port;and wherein individual wavelength channels directed to the M stages aresubject to wavelength selective switching to produce an output at one ofthe 2^(M) output locations.
 2. The optical switching apparatus of claim1 configured as a power monitor.
 3. The optical switching apparatus ofclaim 1 configured to drop selective wavelengths.
 4. The opticalswitching apparatus of claim 1 configured to support node broadcasting.5. The optical switching apparatus of claim 1 further comprising: atleast one optical waveguide to deliver at least one input optical beam;a dispersion device to spatially separate the at least one input opticalbeam into individual wavelength channels; and an optical power device toalign the individual wavelength channels for delivery to the opticalswitch.
 6. The optical switching apparatus of claim 5 furthercomprising: a second optical power device to align the individualwavelength channels from the optical switch; a second dispersion deviceto spatially combine individual wavelength channels from the secondoptical power device; and at least one output optical waveguide toreceive at least one of the individual wavelength channels from thesecond dispersion device.